i didn't know that you could propose to him? when exactly do you get the option to do so? :o
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In order to propose to Cove at the end of Step 3 you need to have the Step 3 DLC, specifically you need to have played the Reflection moment. During that moment ask Cove what his views on marriage are. Just everytime the word marriage comes up you need to hit that option and be positive about it.
And yes, you need to be in love. Don't forget to choose at the start of Step 3 to tell Lee that you're in love with him but haven't admitted it yet.
If you don't have the Step 3 DLC, no worries! You can still propose to Cove yourself in Step 4. Don't have Cove as a fiance, but express a desire to be committed to him someday. Choose to make preparations for your proposal on the second day, this will prompt a dialogue option later in the night to have a proposal in Sunset Bird, which requires you to sneak out. Then finally, all you need to do is actually propose to him on the last day.